Commit message: grep -F: fix a heap buffer (read) overrun

grep's read buffer is often filled to its full size, except when
reading the final buffer of a file.  In that case, the number of
bytes read may be far less than the size of the buffer.  However, for
certain unusual pattern/text combinations, grep -F would mistakenly
examine bytes in that uninitialized region of memory when searching
for a match.  With carefully chosen inputs, one can cause grep -F to
read beyond the end of that buffer altogether.  This problem arose via
commit v2.18-90-g73893ff with the introduction of a more efficient
heuristic using what is now the memchr_kwset function. The use of
that function in bmexec_trans could leave TP much larger than EP,
and the subsequent call to bm_delta2_search would mistakenly access
beyond end of the main input read buffer.

* src/kwset.c (bmexec_trans): When TP reaches or exceeds EP,
do not call bm_delta2_search.
* tests/kwset-abuse: New file.
* tests/Makefile.am (TESTS): Add it.
* THANKS.in: Update.
* NEWS (Bug fixes): Mention it.

Prior to this patch, this command would trigger a UMR:

  printf %0360db 0 | valgrind src/grep -F $(printf %019dXb 0)

  Use of uninitialised value of size 8
     at 0x4142BE: bmexec_trans (kwset.c:657)
     by 0x4143CA: bmexec (kwset.c:678)
     by 0x414973: kwsexec (kwset.c:848)
     by 0x414DC4: Fexecute (kwsearch.c:128)
     by 0x404E2E: grepbuf (grep.c:1238)
     by 0x4054BF: grep (grep.c:1417)
     by 0x405CEB: grepdesc (grep.c:1645)
     by 0x405EC1: grep_command_line_arg (grep.c:1692)
     by 0x4077D4: main (grep.c:2570)

See the accompanying test for how to trigger the heap buffer overrun.

Thanks to Nima Aghdaii for testing and finding numerous
ways to break early iterations of this patch.
CVE: CVE-2015-1345
CWE: CWE-119
Vulnerability description: The bmexec_trans function in kwset.c in grep 2.19 through 2.21 allows local users to cause a denial of service (out-of-bounds heap read and crash) via crafted input when using the -F option.

Project: grep

bmexec_trans (kwset_t kwset, char const *text, size_t size)
{
  unsigned char const *d1;
  char const *ep, *sp, *tp;
  int d;
  int len = kwset->mind;
  char const *trans = kwset->trans;

  if (len == 0)
    return 0;
  if (len > size)
    return -1;
  if (len == 1)
    {
      tp = memchr_kwset (text, size, kwset);
      return tp ? tp - text : -1;
    }

  d1 = kwset->delta;
  sp = kwset->target + len;
  tp = text + len;
  char gc1 = kwset->gc1;
  char gc2 = kwset->gc2;

  /* Significance of 12: 1 (initial offset) + 10 (skip loop) + 1 (md2). */
  if (size > 12 * len)
    /* 11 is not a bug, the initial offset happens only once. */
    for (ep = text + size - 11 * len; tp <= ep; )
      {
        char const *tp0 = tp;
        d = d1[U(tp[-1])], tp += d;
        d = d1[U(tp[-1])], tp += d;
        if (d != 0)
          {
            d = d1[U(tp[-1])], tp += d;
            d = d1[U(tp[-1])], tp += d;
            d = d1[U(tp[-1])], tp += d;
            if (d != 0)
              {
                d = d1[U(tp[-1])], tp += d;
                d = d1[U(tp[-1])], tp += d;
                d = d1[U(tp[-1])], tp += d;
                if (d != 0)
                  {
                    d = d1[U(tp[-1])], tp += d;
                    d = d1[U(tp[-1])], tp += d;

                    /* As a heuristic, prefer memchr to seeking by
                       delta1 when the latter doesn't advance much.  */
                    int advance_heuristic = 16 * sizeof (long);
                    if (advance_heuristic <= tp - tp0)
                      goto big_advance;
                    tp--;
                    tp = memchr_kwset (tp, text + size - tp, kwset);
                    if (! tp)
                      return -1;
                    tp++;
                  }
              }
          }
        if (bm_delta2_search (&tp, ep, sp, len, trans, gc1, gc2, d1, kwset))
          return tp - text;
      big_advance:;
      }

  /* Now we have only a few characters left to search.  We
     carefully avoid ever producing an out-of-bounds pointer. */
  ep = text + size;
  d = d1[U(tp[-1])];
  while (d <= ep - tp)
    {
      d = d1[U((tp += d)[-1])];
      if (d != 0)
        continue;
      if (bm_delta2_search (&tp, ep, sp, len, trans, gc1, gc2, NULL, kwset))
        return tp - text;
    }

  return -1;
}